try to create multi-files Add-on, install fail with Error: module 'bpy' has no attribute 'props'
https://github.com/JacquesLucke/code_autocomplete

Here is a blender add-on that lets you create multi-file add-ons in blender easily. Just download the repository .zip and install the add-on. Next enable it. Now in the text editor click view &gt; properties You will now see a couple new panels one named Addon Development then you will see a text box. Simply type in the name of your new add-on and click the button that pops up that says new addon multi-file Once this is done it will create a multi-file add-on automatically for you. Remember to click save all filesoften and also when you load blender back up and want to work on your add-on again just click the eyedrop tool and you will see a list of all the add-ons you are currently working on and click the one to load and it will. Hope this helps :) 

Also in operator_add.py do you have import bpy? Maybe thats the reason?